1. Field of the Invention
This invention pertains to power supplies for connection to readily available power sources such as 60 Hz, 115 or 230 volts. More specifically, it relates to a self-referencing power converter which senses the current though the primary winding of the power transformer and uses that current ramp for pulse width modulation.
2. Description of the Prior Art
Power supplies typically utilize laminated cores in transformers used to obtain the working voltage or voltages desired. This is accomplished by directly transforming the 60 Hz input voltage. Power supplies utilizing such transformers can be designed to operate quite satisfactorily, but have the disadvantage of large bulk and weight which cannot readily be tolerated for some applications. Associated circuitry, such as filter capacitors, also must be large. To reduce size and weight, unregulated secondary inverters have been provided to supply power. These devices are difficult to regulate. Secondary regulators are typically used, causing a reduction in circuit efficiency and a more expensive power supply.
The switching regulator power supply provides high efficiency and low cost. It commonly incorporates a transformer wound on a ferrite core wherein an energy storage period is used to store energy in the transformer primary and an energy transfer period is used to transfer energy from the primary to the secondary of the transformer when the primary winding is opened. A basic switching regulator power supply monitors electrical parameters in the primary circuit, having an AC coupling only through the transformer to the secondary circuit. Often, a small 60 Hz transformer must be used to assure that the power supply will work upon application of line voltage.
A more advanced switching regulator power supply utilizes a transformer coupled feedback circuit from the secondary circuit to the primary circuit. This permits monitoring secondary power parameters and adjusting the energy storage cycle accordingly, to a degree of accuracy not achievable when the primary circuit only is monitored as is the case of the basic switching regulator power supply.
An example of a sophisticated switching regulator power supply which uses a pulse width modulation technique is described and claimed in U.S. Pat. No. 4,001,663--"Switching Regulator Power Supply". This patent describes the use of optical coupling devices for coupling error signals from the secondary circuit back to the primary circuit where the error signals are utilized to vary the time of the energy storage and energy transfer cycles.
Another type of switching regulator power supply is that described in U.S. Pat. No. 4,092,709 "Multiple Output Self-Oscillating Converter Regulator Power Supply." This patent teaches the varying of the frequency of oscillation of the switching regulator as determined by transformer coupled secondary error signals.
The converter herein described and claimed utilizes a technique of monitoring the current flowing through the primary winding of the power transformer during the energy storage cycle. When that current reaches a preset power reference, a blocking oscillator power switch opens, ending the energy storage cycle. The power reference is set by an error signal from the transformer secondary circuit which is optically coupled to the primary.